IGFETs (insulated gate field effect transistors) may be used as sensors for detection of radiation by detecting a shift in threshold voltage (Vth) or other operating parameter after irradiation. On exposure to radiation, electron hole pairs are generated, and charge is trapped in an insulating layer of the device, e.g. a gate oxide, causing a change in electrical characteristics. Radiation detecting IGFETs, and more particularly silicon MOSFET (metal oxide semiconductor field effect transistor) devices for detection of radiation may be referred to as “RADFETs.”
Conventional RADFETs are active devices, i.e. require application of a relatively high bias, ˜20V, during irradiation to achieve suitable sensitivities (ΔVth of ˜1 mV/rad or more). They also need a thick gate oxide, which requires a custom CMOS process. Moreover, the threshold voltage has a strong dependence on temperature, and suitable compensation is required. As an example, U.S. Pat. No. 4,678,916, to Ian Thomson, entitled “Dosimeter”, discloses the use of matched pairs of silicon MOSFETs. One MOSFET is forward biased during irradiation while operation of the other is inhibited. A measurement of the differential change in threshold voltage between the pair of MOSFETs provides for compensation of threshold drift and offset, and first order compensation of temperature effects.
Floating Gate MOSFET (FGMOSFET) sensors offer some advantages. In particular, the floating gate is pre-charged before irradiation. Thus, the device does not need to be biased during irradiation. Known radiation sensors based on FGMOSFETS are disclosed, for example, in U.S. Pat. No. 6,172,368. entitled “Method of monitoring radiation using a floating gate field effect transistor dosimeter, and dosimeter for use therein”, to N. Garry Tarr and Ian Thomson. For improved sensitivity, this FGMOSFET relies on a large area extended floating gate over a relatively thick dielectric layer, e.g. field oxide, rather than a thick gate oxide. Preferably, the dosimeter comprises a pair of two identical FGMOSFETs with a common source, and each has an extended gate region, a control gate overlying the floating gate, and a charge injector gate. The floating gate of the sensor FGMOSFET is charged prior to irradiation, while the floating gate of the reference FGMOSFET is uncharged or charged with an equal and opposite charge. The dose is measured by monitoring the difference in Vth between the two FGMOSFETs after irradiation.
In more recent work by the same inventors, an article entitled “A sensitive, temperature-compensated, zero-bias floating gate dosimeter”, published in IEEE Transactions on Nuclear Science, vol. 51, no. 3, June 2004, by N. Garry Tarr et al. a FGMOSFET radiation sensor with improved sensitivity is disclosed. The device comprises a FGMOSFET sensor and a reference MOSFET, each having identical channel lengths and channel widths to compensate for temperature effects. Each has a large area extended gate over field oxide. Unlike most FGMOSFET sensors, the floating gate is not overlapped by a control gate, which boosts sensitivity by nearly an order of magnitude. Preferably, a minimum geometry injector gate overlapping the floating gate is provided for precharging the sensor. To protect the device from external electrostatic fields, a grounded electrostatic shielding electrode is provided over the floating gate, e.g. using a Metal 2 layer. Ring shields may also be provided using the first polysilicon and Metal 1 layers. Elimination of a control gate improves sensitivity, because the radiation generated charge is free to concentrate above the FGMOSFET channel. However, this means the charge on the floating gate must be determined from monitoring drain current, rather than a change in threshold voltage. Thus, read-out of the dose requires application of a small bias (˜0.1V) to monitor the drain current. Peak sensitivity of around 3 mV/rad at floating gate potential of −7V was reported in this paper. This reference also discusses in detail cancellation of temperature and environmental effects by use of matched sensor and reference FGMOSFETs and optionally a separate temperature sensor. These FGMOSFET radiation sensors may be fabricated in almost any commercial CMOS process using two polysilicon layers, potentially allowing for integration with read-out and control circuitry.
Nevertheless, there is a need for further improvements in FGMOSFET radiation sensors to improve sensitivity and to allow fabrication using current standard CMOS processes that include only one polysilicon layer, i.e. a “single poly process”, rather than a double polysilicon process. Moreover, although some available sensors are small, each one typically requires wired connections for power and read-out.
Wired connections may cause unpredictable scattering of radiation and for bio-medical applications it is also desirable to free patients and staff from the inconvenience of wired connections for powering and read-out of sensors.
To date, there are a limited number of commercially available wireless radiation sensor modules. Those available tend to be either bulky and/or not capable of real-time read-out or data transmission. For example, a portable, personal electronic dosimeter using an FGMOSFET sensor (DMC2000S) is manufactured by Mirion Technologies (formerly Synodys Inc.) (www.Mirion.com). A “mobileMOSFET” system (TN-RD-70-W) manufactured by Best Medical (www.bestmedicalcanada.com), uses a Bluetooth transmitter/power supply module connected with long wires to one or more MOSFETS sensors. A miniature implantable system with an inductive transmitter is disclosed in U.S. Pat. No. 6,402,689 to Scarantino et al., entitled “Methods, systems, and associated implantable devices for dynamic monitoring of physiological and biological properties of tumors.” Other miniature sensors, as manufactured by Sicel Technologies are disclosed in U.S. patent publication no. 2009/0018403 entitled “Trackable implantable sensor devices systems and related methods of operation”; U.S. Pat. No. 7,557,353 entitled “Single-use external dosimeters for use in radiation therapies” and U.S. Pat. No. 7,495,224 entitled “Single-use external dosimeters for use in radiation therapies and related methods and systems” to Black et al.
In particular, elimination of wired connections during use and wireless real-time read-out would be desirable for bio-medical monitoring, radiotherapy and other applications. However, lower voltage operation and lower power operation is required to facilitate integration of MOSFET radiation sensors with low cost, miniaturized CMOS signal processing circuits and coupling to RF transceivers for wireless operation.